DESCRIPTION: The central goal of this project is an understanding of the fundamental aspects of the normally weak interactions formed between a CH proton donor and an acceptor group containing the electronegative O or N atom. These so-called CH..X (or more commonly CH..O) H-bonds occur in large numbers in biological systems such as proteins and nucleic acids, yet very little is known about even such fundamental features as their strength, and the constraints that their presence places upon the three-dimensional structure of the biomolecule in which they occur. Since crystal structures can say little about the strength of any given interaction, this project turns to ab initio calculations. The formulation of appropriate model systems, coupled with the high levels of accuracy which are now feasible, enables calculations of this sort to elucidate information about these interactions which are simply not attainable by any other means. This project will represent a careful and systematic treatment of CH..X interactions, varying from very weak forces, that differ little from hydrocarbon-hydrocarbon, to those that surpass even conventional H-bonds in strength. The potential of the CH group to participate in a H-bond will be investigated for a range of systems: a) differing in sp(n) hybridization of the C atom, b) with varying proximity of an electronegative group to the CH donor, c) including different acceptor groups, d) groups that have particular biochemical relevance; e.g., the alpha-CH2 group of peptides. The work will search for various properties that may be correlated with one another. Having identified an attractive CH..O interaction, and an associated equilibrium, preferred geometry calculations will determine the sensitivity of the binding energy to stretching and bending deformations; i.e., a potential energy surface will be evaluated. Calculations will address whether cooperativity occurs in CH..X bonds, and if so, to what extent.